The wide range of applications of graphic displays might be the result of the appropriate interconnections of television monitors and computers offered by recent technical progress. There are already a large number of computer controlled graphic displays which are commercially available.
A common feature of such displays lies in that they all have a central clock that controls an address generator to provide horizontal and vertical picture and memory addresses, and the address generator is coupled to a synchronizing unit. The connection towards the computer is established through an interface forming part of the display, and the computer has appropriate access periods in which it can communicate with the random access memory addressed by the address generator and controlled by a memory control unit. A digital to analog converter is coupled to the data bus of the memory and the output PG,4 of the D/A converter provides the standard video signals required for a conventional standard black and white or color television monitor.
In the known apparatuses of the above kind, the computer access is provided during the line returning or picture returning periods.
Since the repetition period of the lines is 64 .mu.s, the computer accesses established during line returning periods are limited in time.
If the computer has limited access time to the memory, the rate of information refreshment or the amount of information that can be read out from the memory in a unit time will be reduced.
For increasing the speed of operation the storage capacity can be increased which, however, is associated with high costs. A substantial part of the cost prices of such apparatuses is the price of the memory elements used therein, therefore the reduction of the number of memory elements is of high importance.
A virtual increase in memory demand in conventional display systems is created by the circumstance that the picture dissolution provided by the television technique can be covered generally by a redundant amount of storage capacity only due to ineffective memory allocation possibilities. This means that the storage capacity of the required number of memory elements is utilized in part only which is associated with decreased storage efficiency.
Of the various types of random access memories, the dynamic RAM memories are considered to be the most favourable when the storage capacity pro unit cost quotient is regarded. For the operation of such dynamic memories, it is required that within as short time periods as about 2 ms, all addresses of the memory should be accessed. With memory addresses corresponding to the vertical and horizontal scanning movement of the electron beam of the monitor, the above condition cannot be satisfied easily. The usage of certain kinds of address modification have already been proposed in connection with the application of random access memories, however, in such cases the modified addresses have been less easy to be handled and inspected than the horizontal and vertical addresses corresponding to the movement of the scanning electron beam which are visual and easy to work with.